Contribution of tundra lakes in western Siberia to the atmospheric methane budget

The emission of methane from tundra lakes in western Siberia is estimated on the basis of experimental data. According to these estimates, over the warm season, the methane flux from the surfaces of lakes to the atmosphere amounts to 0.5 KtCH₄ for Arctic tundra, 2.9 KtCH₄ for typical tundra, and 8.5 KtCH₄ for southern tundra. The short-term spring emission of methane as ice melts in tundra lakes of the region under consideration is determined with consideration for published estimates of the rate of generation of methane in northern-latitude lakes during the cold season. The contribution made by tundra lakes in western Siberia to the atmospheric methane budget is estimated at 20 KtCH₄/year.     

Western Arctic primary productivity regulated by shelf-break warm eddies

The response of phytoplankton to the Beaufort shelf-break eddies in the western Arctic Ocean is examined using the eddy-resolving coupled sea ice–ocean model including a lower-trophic marine ecosystem formulation. The regional model driven by the reanalysis 2003 atmospheric forcing from March to November captures the major spatial and temporal features of phytoplankton bloom following summertime sea ice retreat in the shallow Chukchi shelf and Barrow Canyon. The shelf-break warm eddies spawned north of the Barrow Canyon initially transport the Chukchi shelf water with high primary productivity toward the Canada Basin interior. In the eddy-developing period, the anti-cyclonic rotational flow along the outer edge of each eddy moving offshore occasionally traps the shelf water. The primary production inside the warm eddies is maintained by internal dynamics in the eddy-maturity period. In particular, the surface central area of an anti-cyclonic eddy acquires adequate light, nutrient, and warm environment for photosynthetic activity partly attributed to turbulent mixing with underlying nutrient-rich water. The simulated biogeochemical properties with the dominance of small-size phytoplankton inside the warm eddies are consistent with the observational findings in the western Arctic Ocean. It is also suggested that the light limitation before autumn sea ice freezing shuts down the primary production in the shelf-break eddies in spite of nutrient recovery. These results indicate that the time lag between the phytoplankton bloom in the shelf region following the summertime sea ice retreat and the eddy generation along the Beaufort shelf break is an important index to determine biological regimes in the Canada Basin.     

Remote analysis of changes in the number of small thermokarst lakes and their distribution with respect to their sizes in the cryolithozone of Western Siberia, 2015

Regularities of changes in the number of thermokarst lakes in the cryolithozone of Western Siberia have been studied using multitemporal satellite images. The studies were conducted on 33 test sites selected in different landscape zones of the study area with the use of 134 cloud-free Landsat images obtained during warm months in the period from 1973 to 2013. It is shown that the total number of new lakes is significantly (almost 20 times) higher than the number of disappeared lakes. The area of newly formed thermokarst lakes is, on average, 22 times smaller than the size of disappearing lakes. Therefore, one can assume that the rapid growth in the number of small thermokarst lakes that is observed under the conditions of global warming will lead to an increase in methane emissions into the atmosphere in permafrost zones in the Arctic territories. Results of remote sensing of statistical distribution of small thermokarst lakes with respect to their sizes are given. The studies were performed on eight test sites selected in different permafrost zones of Western Siberia. The size of small lakes was determined according to QuickBird super-high resolution satellite images. It is shown that histograms of distribution of lakes with respect to their sizes allow their approximation by power and exponential functions, which can be used for modeling and predicting the dynamics of thermokarst methane emission in permafrost zones.     

应用木质素示踪楚科奇海表层沉积物中有机碳的来源和降解程度

埋藏在海洋沉积物中的有机碳是大气二氧化碳的净汇,而埋藏过程主要发生在陆架区。北冰洋拥有全球最大的陆架,接收大量来自河流和沿岸侵蚀输运的陆源有机碳,楚科奇海作为北冰洋的边缘海,是有机碳埋藏的重要海区之一。本研究选用楚科奇海和海台的表层沉积物（陆架区33~82.69 m,陆坡区164.63~3 763 m）,通过木质素,结合粒度、比表面积、有机碳百分含量（OC%）、有机碳稳定同位素（δ13C）等指标来示踪楚科奇海沉积物有机碳的来源和降解程度。结果表明,有机碳载荷从陆架到陆坡有明显的降低趋势;δ13C的范围指示有机碳可能来自苔藓、草本裸子植被、浮游植物和冰藻等;木质素丁香基酚（S）与香草基酚（V）的比值（S/V）和肉桂基酚（C）与香草基酚（V）的比值（C/V）表明裸子植物的草本组织贡献了更多的陆源有机碳;此外,较高含量的C9DA二酸（干酪根氧化产物）表明干酪根也可能是楚科奇海表层沉积物中有机质来源的重要组成。指示降解的参数[（Ad/Al） s、（Ad/Al） v、（Ad/Al） p、3,5-Bd/V]在陆架和陆坡沉积物中的差异表明陆坡沉积物中有机质的降解过程受到水动力分选以及原位降解等因素的影响更为明显。     

Contributions to atmospheric methane by natural seepages on the U.K. continental shelf [Mar. Geol. 137 (1997) 165–189]

Natural gas seepages occur on the United Kingdom's continental shelf and although published reports suggest that they are very rare, the petroleum industry has identified, but not publicly reported, many more. There is also very little data on the flux of gas from seabed seepages, and even less on the contribution of seepages to atmospheric concentrations of gases such as methane.

Potential gas source rocks include Quaternary and Tertiary peats as well as petroliferous source rocks such as the Carboniferous Coal Measures and the Upper Jurassic Kimmeridge Clays. There are also other organic-rich sediments which are potential source rocks. Together these cover a considerable part of the U.K. continental shelf.

Analogue seismic reflection (pinger) profiles acquired during the British Geological Survey's regional mapping programme have been reviewed to identify water column targets including fish and plumes of gas bubbles. The ability to distinguish targets is critical to an assessment of the distribution of gas seepages. Both theoretical predictions of target identity and the habits of shoaling fish have been investigated in order to identify a method of distinction.

Data from seabed seepages and measurements of seepage rates have been used to establish likely ranges of gas flux rates and the sizes of gas bubbles. The likelihood that a rising bubble will survive and escape into the atmosphere is determined primarily by bubble size and water depth; methane, the principal constituent of seepage gas, is relatively unreactive and sparingly soluble.

The studies have enabled a new estimate of the distribution of gas seepages on the U.K. continental shelf, and of the contribution to atmospheric methane levels. The results suggest that natural gas seepages are significantly more important as a source of methane than had hitherto been established. It is estimated that between 120,000 and 3.5 mtonnes of methane per year come from a continental shelf area of about 600,000 km². This represents between 2% and 40% of the total United Kingdom methane emission. It is suggested that similar contributions arise from other continental shelf areas worldwide, and that geological sources of atmospheric methane are more significant than is generally acknowledged.     

Convective cloud fields in the Atlantic sector of the Arctic: Satellite and ground-based observations

Convective cloudiness in the Atlantic sector of the Arctic is considered as an atmospheric spatially self-organized convective field. Convective cloud development is usually studied as a local process reflecting the convective instability of the turbulent planetary boundary layer over a heated surface. The convective cloudiness has a different dynamical structure in high latitudes. Cloud development follows cold-air outbreaks into the areas with a relatively warm surface. As a result, the physical and morphological characteristics of clouds, such as the type of convective cloud, and their geographical localization are interrelated. It has been shown that marginal sea ice and coastal zones are the most frequently occupied by Cu hum, Cu med convective clouds, which are organized in convective rolls. Simultaneously, the open water marine areas are occupied by Cu cong, Cb, which are organized in convective cells. An intercomparison of cloud statistics using satellite data ISCCP and ground-based observations has revealed an inconsistency in the cloudiness trends in these data sources: convective cloudiness decreases in ISCCP data and increases in the groundbased observation data. In general, according to the stated hypothesis, the retreat of the sea-ice boundary may lead to an increase in the amount of convective clouds.     

Sources and transport of organic carbon to shelf,slope, and basin surface sediments of the Arctic Ocean

Lipids in surface sediment transects across the Arctic Ocean were identified to define the sources of organic carbon and the transport of material in the ocean basin. Sterols representing diatoms (24-methylcholesta-5,24(28)-dien-3β-ol, 24-methylcholesta-5,22-dien-3β-ol) and dinoflagellates (4α,23,24-trimethylcholest-22-en-3β-ol) together with algal polyunsaturated fatty acids (20:5, 22:6) demonstrated the importance of primary production to organic matter inputs on the Chukchi Shelf. The presence of terrestrial biomarkers including long-chain n-alkanes and mono- and dicarboxylic acids in shelf sediments indicated that while the fraction of terrestrial biomarkers was small compared to marine material, the transport of allochthonous material impacts carbon cycling on the shelf. Algal biomarkers were found in all surficial sediments from the central Arctic basins, demonstrating that some fraction of primary production reached bottom sediments despite ice cover and light limitation. Marine markers represented a small fraction of the total lipids in central basin sediments. This implies that the basins are less productive than shallow waters, significant degradation occurs before the organic matter reaches the sediment–water interface, and substantial amounts of vascular plant material are exported to the central Arctic. Circulation and topographical features, such as the Transpolar Drift and the Lomonosov Ridge, appear to have an important influence on the transport and focusing of terrestrial material in the Arctic Ocean basins.     

西伯利亚极地海域表层沉积硅藻分布特征及其与海洋环境变量的相关性研究

北冰洋西伯利亚陆架海是北极气候快速变化最为显著的海域之一,而沉积硅藻作为极地海洋生态系统的重要组成部分,对环境变化具有敏感的响应。对楚科奇海、东西伯利亚海和拉普捷夫海表层沉积物开展了硅藻组成鉴定,利用典型对应分析方法分析了硅藻属种与1986～2015年环境变量之间的关系。结果表明,夏季和秋季海冰密集度、表层海水盐度是影响研究区表层硅藻分布特征最主要的因素。此外,根据表层站位与环境变量的典型对应分析,可将西伯利亚极地海域划分为4个区域,分别为海冰硅藻组合带、暖水硅藻组合带、沿岸硅藻组合带和混合硅藻组合带。这些表层站位的分区与相应区域的海流模式有明显的相关性,海冰硅藻组合带仅分布于研究区北部的高纬度地区;暖水硅藻组合带位于受白令水和太平洋海水的分支——阿拉斯加沿岸水影响为主的区域;拉普捷夫海南部的沿岸硅藻组合带则受到河流径流和西伯利亚沿岸流的强烈影响;混合硅藻组合带受极地冷水、海冰覆盖、太平洋暖水和陆地径流的共同影响。     

巴芬湾西部和格陵兰岛东部大气经向热量输送协同变化对夏季北极海冰的影响

利用欧洲中心气候再分析资料和美国国家冰雪数据中心北极海冰面积资料,分析了夏季北极海冰面积与前期大气经向热量输送年际变化的联系。结果表明：6月北半球中高纬大气的经向热量输送以瞬变热量形式为主,其中巴芬湾西部（B区）和格陵兰岛东部（G区）是瞬变热量向极区传输的两个通道,二者之间存在反位相的协同变化,且这种协同变化与夏季北极海冰面积变化密切相关。可能的机制为：6月,AD、AO和NAO三种北极大气环流型能够引起巴芬湾西部和格陵兰岛东部瞬变热量输送的协同变化,这种协同变化通过涡旋动力作用激发夏季极区大气表现为AD异常,同时影响途经区域的气温,从而通过热动力作用影响夏季北极海冰。将向极区输送的热量称为暖输送,从极区输出的热量为冷输送,则上述两个区域的瞬变热量协同输送可分为三种情况：B暖G冷、B冷G暖、B和G均冷,而B和G均暖的情况十分罕见。当B区向极区输入、G区输出热量时,有利于太平洋扇区和喀拉海的海冰偏少;当G区输入、B区输出热量时,利于喀拉海和拉普捷夫海海冰偏少;当B区和G区均输出热量时,利于波佛特海南部、喀拉海和拉普捷夫海海冰偏多,反之则相反。     

Trace element and molecular markers of organic carbon dynamics along a shelf–basin continuum in sediments of the western Arctic Ocean

Molecular organic biomarkers together with trace element composition were investigated in sediments east of Barrow Canyon in the western Arctic Ocean to determine sources and recycling of organic carbon in a continuum from the shelf to the basin. Algal biomarkers (polyunsaturated and short-chain saturated fatty acids, 24-methylcholesta-5,24(28)-dien-3β-ol, dinosterol) highlight the substantial contribution of organic matter from water column and sea-ice primary productivity in shelf environments, while redox markers such as acid volatile sulfide (AVS), Mn, and Re indicate intense metabolism of this material leading to sediment anoxia. Shelf sediments also receive considerable inputs from terrestrial organic carbon, with biomarker composition suggesting the presence of multiple pools of terrestrial organic matter segregated by age/lability or hydrodynamic sorting. Sedimentary metabolism was not as intense in slope sediments as on the shelf; however, sufficient labile organic matter is present to create suboxic and anoxic conditions, at least intermittently, as organic matter is focused towards the slope. Basin sediments also showed evidence for episodic delivery of labile organic carbon inputs despite the strong physical controls of water depth and sea-ice cover. Principal components analysis of the lipid biomarker data was used to estimate fractions of preserved recalcitrant (of terrestrial origin) and labile (of marine origin) organic matter in the sediments, with ranges of 12–79%, 14–45%, and 37–66% found for the shelf, slope, and basin cores, respectively. On average, the relative preserved terrestrial organic matter in basin sediments was 56%, suggesting exchange of organic carbon between nearshore and basin environments in the western Arctic.     

Cooling in the western Bering Sea in 1999: quick propagation of La Niña signal or compensatory processes effect?

In 1999, synoptic and hydrological conditions in the western Bering Sea were characterized by negative SST and air temperature anomalies, extensive ice coverage and late melting. Biological processes were also delayed. In 1999, the average zooplankton biomass was 1.76 g/m³, approximately half the average 3.07 g/m³ in 1998. Pacific salmon migrated to the northeastern Kamchatka streams two weeks later. This contrasts with 1997 (spring and summer) and 1998 (summer) when positive SST anomalies were widely distributed throughout the northwestern Bering Sea shelf. Since the second half of the 1990s, seasonal atmospheric processes developed over the western Bering Sea that were similar to those of the cold decades of the 1960–1970s. A meridional atmospheric circulation pattern began to replace zonal transport. Colder Arctic air masses have shifted over the Bering Sea region and shelf water temperatures have cooled considerably with the weakening of zonal atmospheric circulation. Temperature decreased in the cold intermediate layer during its renewal in winter. Besides, oceanic water inflow intensified into the Bering Sea in intermediate layers. Water temperature warmed to 4°C and a double temperature maximum existed in the warm intermediate layer in late summer in both 1997 and 1998. Opposing trends of cold water temperature and a warm intermediate layer led to an increase of vertical gradients in the main thermocline and progressing frontogenesis. It accelerates frontal transport and can be regarded as a chief cause of increased water exchange with the Pacific Ocean.     

Some features of seasonal variations in the methane content in the atmosphere over northern Eurasia

The climatic trends and basic features of seasonal variations in and anomalies of the concentration of methane in the atmospheric surface layer are considered on the basis of the current notion of the processes that form the global field of methane in the Earth’s atmosphere. Measurement data on the surface concentration of methane, which were obtained in Moscow and at a number of observation stations in Europe and Siberia in the fall-winter period of the first decade of the 21st century, have been analyzed. It is shown that, in the anomalously warm winter months of 2006/2007, the concentration of methane in the atmosphere over Moscow was higher than in the previous and following years. The excess concentration of methane amounted to 10% in March 2007, which is higher than the mean range of seasonal variations in the monthly mean concentration of surface methane. A comparison between the data obtained in Moscow and the data obtained at three stations of the NOAA global monitoring network and at three Russian Hydrometeorological Research Center stations shows the high spatial variability of the methane concentration in the atmosphere over northern Eurasia. The complex and multifactor processes that determine the content of methane in the atmospheric surface layer result in noticeable spatial and interannual deviations from the mean seasonal cycle of its concentration, which can manifest themselves on both regional and global scales. It is possible that the resumed increase in the content of methane in the Earth’s atmosphere recorded in 2007 (after its relative stabilization in the early 2000s) at the global monitoring network was also caused, to some extent, by the anomalously warm winter of 2006–2007 in northern Europe and western Siberia.     

东海海水中的溶存甲烷

基于1994年秋季航次在东海的调查资料,较详细地分析了溶存甲烷在水体中的分布规律、成因和来源.表层水中溶存甲烷呈过饱和状态,饱和度127%～254%,温跃层以上水体中,甲烷的断面分布不同于营养盐的分布,各站测值相近,没有显示出受长江冲淡水的影响,而呈现的舌状分布,在陆架底层水中有明显的高浓度甲烷水体,表明甲烷从沉积物中迅速扩散进入底层水.黑潮次表层水的涌升过程稀释了陆架边缘底层水中的甲烷.在陆架和大洋区测站上,甲烷的垂直分布不同,前者主要受物理混合过程所控制;后者呈大洋区分布特征,在温跃层附近出现甲烷的次表层最大,这可能是陆架底层高浓度甲烷沿等密度面的输送所致.     

The Western Arctic Shelf–Basin Interactions (SBI) project,volume II: An overview

The Western Arctic Shelf–Basin Interactions (SBI) project is a 10-year Arctic environmental change program. A major field campaign occurred in the spring, summer and fall seasons between 2002 and 2004. The SBI program was developed to investigate the production, transformation and fate of carbon on shelf and slope regions of the Chukchi and Beaufort seas and shelf–basin interactions with the Arctic Basin. This 2nd special issue documents many of the key findings, ranging from sea ice and hydrographic changes to water column and sediment dynamics and trophic level interactions during the core Phase II field program. The integration of these results with ongoing synthesis and modeling activities within the SBI program are illustrating the critical importance of this continental shelf environment to understand shelf–basin interactions and the ongoing changes being observed and forecasted in the marine Amerasian Arctic.     

北冰洋太平洋扇区碳循环过程对环境变化响应的研究进展与展望

全球碳增汇需求高涨,海冰消退后的北冰洋被期待是一个主要的潜在碳增汇区。北冰洋太平洋扇区因受控于楚科奇海及其邻近海域较高的海洋固碳效率和碳深海封存量,在整个北冰洋碳循环中起着举足轻重的作用。开展该海域碳循环过程对环境快速变化的响应机制研究是实现北冰洋碳汇精准预测的基础。本文重点阐述了楚科奇海及其邻近海域碳循环过程（即海洋对大气二氧化碳的吸收、生物固碳、太平洋入流携带碳经陆架生物地化过程后向深海输出封存的陆架泵）对北冰洋环境快速变化的响应,并提出未来研究需要聚焦的关键科学问题。     

Co-variation of the surface wind speed and the sea surface temperature over mesoscale eddies in the Gulf Stream region:momentum vertical mixing aspect

The co-variation of surface wind speed and sea surface temperature (SST) over the Gulf Stream frontal region is investigated using high-resolution satellite measurements and atmospheric reanalysis data. Results show that the pattern of positive SST-surface wind speed correlations is anchored by strong SST gradient and marine atmospheric boundary layer (MABL) height front, with active warm and cold-ocean eddies around. The MABL has an obvious transitional structure along the strong SST front, with greater (lesser) heights over the north (south) side. The significant positive SST-surface wind-speed perturbation correlations are mostly found over both strong warm and cold eddies. The surface wind speed increases (decreases) about 0.32 (0.41) m/s and the MABL elevates (drops) approximate 55 (54) m per 1℃ of SST perturbation induced by warm (cold) eddies. The response of the surface wind speed to SST perturbations over the mesoscale eddies is mainly attributed to the momentum vertical mixing in the MABL, which is confirmed by the linear relationships between the downwind (crosswind) SST gradient and wind divergence (curl).     

Dense water formation and circulation in the Barents Sea

Dense water masses from Arctic shelf seas are an important part of the Arctic thermohaline system. We present previously unpublished observations from shallow banks in the Barents Sea, which reveal large interannual variability in dense water temperature and salinity. To examine the formation and circulation of dense water, and the processes governing interannual variability, a regional coupled ice-ocean model is applied to the Barents Sea for the period 1948-2007. Volume and characteristics of dense water are investigated with respect to the initial autumn surface salinity, atmospheric cooling, and sea-ice growth (salt flux). In the southern Barents Sea (Spitsbergen Bank and Central Bank) dense water formation is associated with advection of Atlantic Water into the Barents Sea and corresponding variations in initial salinities and heat loss at the air-sea interface. The characteristics of the dense water on the Spitsbergen Bank and Central Bank are thus determined by the regional climate of the Barents Sea. Preconditioning is also important to dense water variability on the northern banks, and can be related to local ice melt (Great Bank) and properties of the Novaya Zemlya Coastal Current (Novaya Zemlya Bank). The dense water mainly exits the Barents Sea between Frans Josef Land and Novaya Zemlya, where it constitutes 63% (1.2 Sv) of the net outflow and has an average density of 1028.07 kg m⁻³. An amount of 0.4 Sv enters the Arctic Ocean between Svalbard and Frans Josef Land. Covering 9% of the ocean area, the banks contribute with approximately 1/3 of the exported dense water. Formation on the banks is more important when the Barents Sea is in a cold state (less Atlantic Water inflow, more sea-ice). During warm periods with high throughflow more dense water is produced broadly over the shelf by general cooling of the northward flowing Atlantic Water. However, our results indicate that during extremely warm periods (1950s and late 2000s) the total export of dense water to the Arctic Ocean becomes strongly reduced.     

白令海陆架区夏季水文分布特征及年际变化

On the basis of the CTD data obtained within the Bering Sea shelf by the Second to Sixth Chinese National Arctic Research Expedition in the summers of 2003, 2008, 2010, 2012 and 2014, the classification and interannual variation of water masses on the central Bering Sea shelf and the northern Bering Sea shelf are analyzed. The results indicate that there are both connection and difference between two regions in hydrological features. On the central Bering Sea shelf, there are mainly four types of water masses distribute orderly from the slope to the coast of Alaska: Bering Slope Current Water(BSCW), MW(Mixed Water), Bering Shelf Water(BSW) and Alaska Coastal Water(ACW). In summer, BSW can be divided into Bering Shelf Surface Water(BSW_S) and Bering Shelf Cold Water(BSW_C). On the northern Bering Sea shelf near the Bering Strait,it contains Anadyr Water(AW), BSW and ACW from west to east. But the spatial-temporal features are also remarkable in each region. On the central shelf, the BSCW is saltiest and occupies the west of 177°W, which has the highest salinity in 2014. The BSW_C is the coldest water mass and warmest in 2014; the ACW is freshest and mainly occupies the east of 170°W, which has the highest temperature and salinity in 2012. On the northern Bering Sea shelf near the Bering Strait, the AW is saltiest with temperature decreasing sharply compared with BSCW on the central shelf. In the process of moving northward to the Bering Strait, the AW demonstrates a trend of eastward expansion. The ACW is freshest but saltier than the ACW on the central shelf,which is usually located above the BSW and is saltiest in 2014. The BSW distributes between the AW and the ACW and coldest in 2012, but the cold water of the BSW_C on the central shelf, whose temperature less than 0°C, does not exist on the northern shelf. Although there are so many changes, the respond to a climate change is synchronized in the both regions, which can be divided into the warm years(2003 and 2014) and cold years(2008, 2010 and 2012). The year of 2014 may be a new beginning of warm period.     

Warming and depth convergence of the Arctic Intermediate Water in the Canada Basin during 1985-2006

The warming of the Arctic Intermediate Water (AIW) is studied based on the analyses of hydrographic observations in the Canada Basin of the Arctic Ocean during 1985-2006. It is shown that how the anomalously warm AIW spreads in the Canada Basin during the observation time through the analysis of the AIW temperature spatial distribution in different periods. The results indicate that by 2006, the entire Canada Basin has almost been covered by the warming AIW. In order to study interannual variability of the AIW in the Canada Basin, the Canada Basin is divided into five regions according to the bottom topography. From the interannual variation of AIW temperature in each region, it is shown that a cooling period follows after the warming event in upstream regions. At the Chukchi Abyssal Plain and Chukchi Plateau, upstream of the Arctic Circumpolar Boundary Current (ACBC) in the Canada Basin, the AIW temperature reached maximum and then started to fall respectively in 2000 and 2002. However, the AIW in the Canada Abyssal Plain and Beaufort Sea continues to warm monotonically until the year 2006. Furthermore, it is revealed that there is convergence of the AIW depth in the five different regions of the Canada Basin when the AIW warming occurs during observation time. The difference of AIW depth between the five regions of the Canada Basin is getting smaller and smaller, all approaching 410 m in recent years. The results show that depth convergence is related to the variation of AIW potential density in the Canada Basin.     

Inter-decadal variability in zooplankton and phytoplankton abundance on the Newfoundland and Scotian shelves

Continuous Plankton Recorder (CPR) sampling on the Newfoundland and Scotian shelves covers three multi-year periods characterised by negative (1962–1971), positive (1992–2000) and negative/neutral (2001–2003) values of the NAO index. Water temperatures respond differently to changes in the NAO in different regions: a positive NAO index tends to lead to reduced temperatures on the Newfoundland shelf and to increased temperatures on the central/western Scotian shelf, and a negative NAO index to the reverse. Since the 1960s, the hydrographic changes due to the NAO have been superimposed on a freshening of the water column throughout the region, which is attributed to increased contribution of Arctic water outflow. Changes in plankton abundance measured by the CPR for the three time periods were generally, but not always, similar on the Newfoundland and eastern and western regions of the Scotian shelf, although Arctic species (e.g. Calanus glacialis, Calanus hyperboreus) were notably more abundant and warm water species (e.g. Metridia lucens, euphausiids) less abundant on the Newfoundland shelf than on the Scotian shelf. Three categories of phytoplankton (colour, diatoms, dinoflagellates) increased in abundance in the 1990s, and these increases generally persisted into 2001–2003. This is believed to be a response to the persistent freshening of the water column, probably due to increased stratification. The Arctic species C. glacialis and C. hyperboreus also showed persistent increases in abundance after 1992, perhaps due to increased transport from the Arctic, although the abundance of the Arctic slope water species Metridia longa decreased. Two groups, Calanus 1–4 and euphausiids, both thought to play important roles in the food chain, showed persistent decreases in abundance after 1992, especially on the Newfoundland shelf. In all regions, Calanus finmarchicus 5–6, Oithona spp. and Centropages hamatus abundance changed in association with variations in the NAO, although no common mechanism could be identified. C. finmarchicus 5–6 abundance decreased in the 1990s and increased after 2001, while the other two species showed the opposite pattern. Centropages typicus and M. lucens abundance on the Scotian shelf increased with rising temperature. This is attributed to increased production rates for the former and an increased influx of warm, M. lucens-rich, slope water on to the shelf for the latter. A comparison between ring net and CPR sampling on the Newfoundland shelf suggests that the Calanus 1–4 category is dominated by C. finmarchicus and that late stage C. glacialis and C. hyperboreus are grossly under-sampled compared to late stage C. finmarchicus.